The polymicrobial nature of the skin is illustrated by the isolation of both Gram-negative and Gram-positive bacteria, which altogether seem to constitute a natural barrier for opportunistic or pathogenic microbes. Bacterial cells communicate with one another by quorum sensing mechanisms, through the secretion of signaling organic molecules such as derivatives of homoserine lactones in Gram-negative bacteria and proteins and peptides in Gram-positive bacteria to activate or suppress functions necessary for their survival. As a model system for the study of the effect of quorum sensing on bacterial interaction, we will examine the effect(s) of RIP, a quorum sensing heptapeptide produced by Staphylococcus xylosus, on the survival and virulence of S. aureus and S. epidermidis. S. aureus and S. epidermidis are part of the normal flora of the human skin but can cause fatal diseases when their density increases, probably due to the expression of toxic exomolecules or due to the formation of biofilms. The expression of toxic exomolecules as well as biofilm formation can be inhibited by RIP, which interferes with staphylococcal quorum sensing mechanisms. RIP does not kill the bacteria but keeps them in a non-pathogenic stage, suggesting that the natural composition of the normal flora would not be effected. To further develop RIP as a therapeutic peptide, it is important to understand its molecular targets and to test its effect(s) on the normal flora in a polymicrobial environment. To achieve these goals, we propose to determine the molecular targets of RIP in S. aureus and S. epidermidis by functional genomics and proteomics and, funding and time permitting, to test its effects in vivo on the survival of a polymicrobial inoculum of both S. aureus and S. epidermidis, using the murine air sac model. Using the same in vivo model, we will also test the effect of S. xylosus that naturally produces RIP as a probiotic to prevent staphylococcal pathogenesis. [unreadable] [unreadable]